QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 575
24-BIT DIFFERENTIAL INPUT DELTA SIGMA ADC
LTC2410
DESCRIPTION
Demonstration circuit 575 features the LTC2410, a
24-bit high performance
∆Σ
analog-to-digital con-
verter (ADC). The LTC2410 features 2ppm linearity,
2.5ppm full-scale accuracy, 0.1ppm offset, and
0.16ppm noise. The inputs and reference are fully
differential, with input common mode rejection of 140
dB. The LTC2410 is available in a 16 pin GN package
and has an easy to use SPI interface.
DC575 is a member of Linear Technology‘s
QuickEval™ family of demonstration boards. It is de-
signed to allow easy evaluation of the LTC2410 and
may be connected directly to the target application’s
analog signals while using the DC590 USB Serial
Controller board and supplied software to measure
performance. The exposed ground planes allow
proper grounding to prototype circuitry. After evaluat-
ing with Linear Technology’s software, the digital sig-
nals can be connected to the end application’s proc-
essor / controller for development of the serial inter-
face.
Design files for this circuit board are available. Call
the LTC factory.
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QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 575
24-BIT DIFFERENTIAL INPUT DELTA SIGMA ADC
QUICK START PROCEDURE
Connect DC575 to a DC590 USB Serial Controller us-
ing the supplied 14-conductor ribbon cable. Connect
DC590 to the host PC with a standard USB A/B cable.
Run the evaluation software supplied with DC590 or
downloaded from
www.linear.com.
The correct pro-
gram will be loaded automatically. Click the COLLECT
button to start reading the input voltage. Details on
software features are documented in the control
panel’s help menu.
Tools are available for logging data, changing refer-
ence voltage, changing the number of points in the
strip chart and histogram, and changing the number
of points averaged for the DVM display.
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QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 575
24-BIT DIFFERENTIAL INPUT DELTA SIGMA ADC
HARDWARE SET-UP
JUMPERS
JP1 – Select the source for REF+, either external or
5.00 volts from the onboard LT1236 reference (de-
fault.)
JP2 – Select the source for REF-, either external or
Ground (0 volts, default.)
JP3 – Select the trigger mode, either normal (de-
fault) or externally triggered.
JP4 – Select the notch frequency. This selects ei-
ther 50Hz or 60Hz rejection mode. Remove this
jumper when supplying an external clock to the Fo
turret.
JP5 – Trigger input signal. Pin 1 is a 5 volt logic
signal, pin 2 is ground. When triggered mode is
selected on JP3, a rising edge starts a new conver-
sion.
CONNECTION TO DC590 SERIAL CONTROLLER
J2 is the power and digital interface connector.
Connect to DC590 serial controller with supplied
14-conductor ribbon cable.
An external conversion clock may be applied to the
Fo turret to modify the frequency rejection charac-
teristics or data output rate of the LTC2410. Be sure
to remove JP4 before applying an external clock.
This should be a square wave with a low level equal
to ground and a high level equal to Vcc. While up to
a 2MHz clock can be used, performance may be
compromised. Refer to the LTC2410 data sheet.
ANALOG CONNECTIONS
Analog signal connections are made via the row of
turret posts along the edge of the board. When
connecting the board to an existing circuit, the ex-
posed ground planes along the edges of the board
may be used to form a solid connection between
grounds.
GND – Ground turrets are connected directly to the
internal ground planes.
VCC – This is the supply for the ADC. Do not draw
any power from this point.
REF+, REF- – These are connected to the LTC2410
REF+ and REF- pins. If the onboard reference is be-
ing used, the reference voltage may be monitored
from this point. An external reference may be con-
nected to these terminals if JP1 and JP2 are con-
figured for external reference.
IN+, IN- – These are the differential inputs to the
LTC2410.
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QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 575
24-BIT DIFFERENTIAL INPUT DELTA SIGMA ADC
EXPERIMENTS
INPUT NOISE
Solder a short wire from the IN- turret post to the
IN+ turret post. Noise should be approximately
0.16ppm (800nV.)
COMMON MODE REJECTION
Tie the two inputs (still connected together) to
ground through a short wire and note the indicated
voltage. Tie the inputs to REF+; the difference
should be less than 1.6uV due to the 130dB CMRR
of the LTC2410.
RESOLVING
SHUNTS
MILLIAMPS
WITH
MILLIOHM
BIPOLAR SYMMETRY
To demonstrate the symmetry of the ADCs transfer
function, connect a stable, low noise, floating volt-
age source (with a voltage less than Vref/2) from
IN+ to IN- and note the indicated voltage. Reverse
the polarity; the indicated voltage will typically be
within 15uV of the first reading, multiplied by –1.
One convenient voltage source for this experiment
is a single alkaline battery. While a battery has fairly,
low noise, it is sensitive to temperature drift. It is
best to use a large (D-size) battery that is insulated
from air currents. A better source is a battery pow-
ered series reference such as the LT1790. This part
is available with output voltages of 1.25V, 2.048V,
2.5V, 3V, 3.3V, 4.096V and 5V.
INPUT NORMAL MODE REJECTION
The LTC2410’s SINC4 digital filter is trimmed to
strongly reject 50 or 60Hz line noise when operated
with the internal conversion clock. To measure in-
put normal mode rejection, connect IN- to a 2.5 volt
source such as an LT1790-2.5 reference or a power
supply. Apply a 10Hz, 2V peak-to-peak sine wave to
IN+ through a 1uF capacitor.
Start taking data. The input noise will be quite large,
and the graph of output vs. time should show large
variations.
Next, slowly increase the frequency to 60Hz. The
noise should be almost undetectable in the graph.
Note that the indicated noise in ppm may still be
above that of the datasheet specification because
the inputs are not connected to a DC source.
One application that can benefit greatly from the
LTC2410’s input resolution is current measurement.
It is advantageous to use a very low resistance
shunt to minimize the voltage drop. To demonstrate
this, make a simple current shunt by soldering a 1-
inch length of 24 gauge copper wire from the IN+
turret to the IN- turret. This is a resistance of ap-
proximately 2.4 milliohms. Connect the IN- turret to
the GND turret through a short wire. Start the dem-
onstration software and note the initial voltage,
which should be close to zero. Next, connect IN+ to
REF+ through a 1k resistor, which will allow ap-
proximately 5mA to flow through the shunt. The
indicated voltage should increase by approximately
12uV (The actual increase will depend on the toler-
ance of the wire material, diameter, and length.)
Since the common mode range of the inputs ex-
tends from ground to VCC, the current shunt can
also be used at the “high side.” To do this, tie the
IN+ turret to VCC and connect the resistor from IN-
to ground. Thus, the current of a circuit can be
monitored with minimal impact on supply voltage
and without breaking any ground connections.
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QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 575
24-BIT DIFFERENTIAL INPUT DELTA SIGMA ADC
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